Methanol carbonylation process

ABSTRACT

An alcohol such as methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide, along with alkyl iodide such as methyl iodide and alkyl acetate such as methyl acetate in specified proportions. With a finite concentration of water in the reaction medium the product is the carboxylic acid instead of, for example, the anhydride. The present reaction system not only provides an acid product of unusually low water content at unexpectedly favorable reaction rates but also, whether the water content is low or, as in the case of prior-art acetic acid technology, relatively high, is characterized by unexpectedly high catalyst stability; i.e., it is resistant to catalyst precipitation out of the reaction medium.

RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 870,267, filed Jun. 3,1986, U.S. Pat. No. 5,001,259, which is a continuation-in-part of U.S.application Ser. No. 699,525, filed Feb. 8, 1985, abandoned which is acontinuation-in-part of application Ser. No. 606,730, filed May 3, 1984,now abandoned.

BACKGROUND OF THE INVENTION AND PERTINENT PRIOR ART

Among currently-employed processes for synthesizing acetic acid one ofthe most useful commercially is the carbonylation of methanol withcarbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik etal. on Oct. 30, 1973. The catalyst comprises rhodium, either dissolvedor otherwise dispersed in the liquid reaction medium or else supportedon an inert solid, along with a halogen-containing catalyst promoter asexemplified by, for example, methyl iodide. The rhodium can beintroduced into the reaction system in any of many forms, and it is notrelevant, if indeed it is possible, to identify the exact nature of therhodium moiety within the active catalyst complex. Likewise, the natureof the halide promoter is not critical. The patentees disclose a verylarge number of suitable promoters, most of which are organic iodides.These compounds are employed as promoters, not stabilizers. Mosttypically and usefully, the reaction is conducted with the catalystbeing dissolved in a liquid reaction medium through which carbonmonoxide gas is continuously bubbled.

Paulik et al. teach that the liquid reaction medium can be any solventcompatible with the catalyst system and that it may comprise, forexample, the pure alcohol which is being reacted, or mixtures thereofwith the desired carboxylic acid end product and/or esters of these twocompounds. However, the patentees teach further that the preferredsolvent and liquid reaction medium for the process is the desiredcarboxylic acid itself, i.e., acetic acid when methanol is beingcarbonylated to produce acetic acid.

An important aspect of the teachings of Paulik et al. is that watershould also be present in the reaction mixture in order to attain asatisfactorily high reaction rate. The patentees exemplify a largenumber of reaction systems including a large number of applicable liquidreaction media. The general thrust of their teachings is, however, thata substantial quantity of water helps in attaining an adequately highreaction rate. The patentees teach furthermore that reducing the watercontent leads to the production of ester product as opposed tocarboxylic acid. Considering specifically the carbonylation of methanolto acetic acid in a solvent comprising predominantly acetic acid andusing the promoted catalyst taught by Paulik et al., it is taught inEuropean Patent Application 0055 618 that typically about 14-15 wt%water is present in the reaction medium of a typical acetic acid plantusing this technology. It will be seen that in recovering acetic acid inanhydrous or nearly anhydrous form from such a reaction solvent,separating the acetic acid from this appreciable quantity of water,involves substantial expenditure of energy in distillation and/oradditional processing steps such as solvent extraction, as well asenlarging some of the process equipment as compared with that used inhandling drier materials. Also Hjortkjaer and Jensen [Ind. Eng. Chem.,Prod Res. Dev. 16, 281-285 (1977)] have shown that increasing the waterfrom 0 to 14 wt% water increases the reaction rate of methanolcarbonylation. Above 14 wt% water the reaction rate is unchanged.

In addition, as will be further explained hereinbelow, the catalysttends to precipitate out of the reaction medium as employed in theprocess of Paulik et al., especially during the course of distillationoperations to separate the product from the catalyst solution when thecarbon monoxide content of the catalyst system is reduced (EP0055618).It has now been found that this tendency increases as the water contentof the reaction medium is decreased. Thus, although it might appearobvious to try to operate the process of Paulik et al. at minimal waterconcentration in order to reduce the cost of handling reaction productcontaining a substantial amount of water while still retaining enoughwater for adequate reaction rate, the requirement for appreciable waterin order to maintain catalyst activity and stability works against thisend.

Other reaction systems are known in the art in which an alcohol such asmethanol or an ether such as dimethyl ether can be carbonylated to anacid or ester derivative using special solvents such as aryl esters ofthe acid under substantially anhydrous reaction conditions. The productacid itself can be a component of the solvent system. Such a process isdisclosed in U.S. Pat. No. 4,212,989 issued Jul. 15, 1980 to Isshiki etal., with the catalytic metal being a member of the group consisting ofrhodium, palladium, iridium, platinum, ruthenium, osmium, cobalt, iron,and nickel. A somewhat related patent is U.S. Pat. No. 4,336,399 to thesame patentees, wherein a nickel-based catalyst system is employed.Considering U.S. Pat. No. 4,212,989 in particular, the relevance to thepresent invention is that the catalyst comprises both the catalyticmetal, as exemplified by rhodium, along with what the patenteescharacterize as a promoter, such as the organic iodides employed byPaulik et al. as well as what the patentees characterize as an organicaccelerating agent. The accelerating agents include a wide range oforganic compounds of trivalent nitrogen, phosphorus, arsenic, andantimony Sufficient accelerator is used to form a stoichiometriccoordination compound with the catalytic metal. Where the solventconsists solely of acetic acid, or acetic acid mixed with the feedstockmethanol, only the catalyst promoter is employed (without theaccelerating agent), and complete yield data are not set forth. It isstated, however, that in this instance "large quantities" of water andhydrogen iodide were found in the product, which was contrary to theintent of the patentees.

European Published Patent Application No. 0 055 618 to Monsanto Companydiscloses carbonylation of an alcohol using a catalyst comprisingrhodium and an iodine or bromine component wherein precipitation of thecatalyst during carbon monoxide-deficient conditions is alleviated byadding any of several named stabilizers. A substantial quantity ofwater, of the order of 14-15 wt%, was employed in the reaction medium.The stabilizers tested included simple iodide salts, but the moreeffective stabilizers appeared to be any of several types ofspecially-selected organic compounds. There is no teaching that theconcentrations of methyl acetate and iodide salts are significantparameters in affecting the rate of carbonylation of methanol to produceacetic acid especially at low water concentrations. When an iodide saltis used as the stabilizer, the amount used is relatively small and theindication is that the primary criterion in selecting the concentrationof iodide salt to be employed is the ratio of iodide to rhodium. Thatis, the patentees teach that it is generally preferred to have an excessof iodine over the amount of iodine which is present as a ligand withthe rhodium component of the catalyst. Generally speaking the teachingof the patentees appears to be that iodide which is added as, forexample, an iodide salt functions simply as a precursor component of thecatalyst system. Where the patentees add hydrogen iodide, they regard itas a precursor of the promoter methyl iodide. There is no clear teachingthat simple iodide ions as such are of any significance nor that it isdesirable to have them present in substantial excess to increase therate of the reaction. As a matter of fact Eby and Singleton [AppliedIndustrial Catalysis, Vol. 1, 275-296(1983)] from Monsanto state thatiodide salts of alkali metals are inactive as cocatalyst in therhodium-catalyzed carbonylation of methanol.

Carbonylation of esters, such as methyl acetate, or ethers, such asdimethyl ether, to form a carboxylic acid anhydride such as aceticanhydride is disclosed in U.S. Pat. No. 4,115,444 to Rizkalla and inEuropean Patent Application No. 0,008,396 by Erpenbach et. al. andassigned to Hoechst. In both cases the catalyst system comprisesrhodium, an iodide, and a trivalent nitrogen or phosphorus compound.Acetic acid can be a component of the reaction solvent system, but it isnot the reaction product. Minor amounts of water are indicated to beacceptable to the extent that water is found in thecommercially-available forms of the reactants. However, essentially dryconditions are to be maintained in these reaction systems.

U.S. Pat. No. 4,374,070 to Larkins et al. teaches the preparation ofacetic anhydride in a reaction medium which is, of course, anhydrous bycarbonylating methyl acetate in the presence of rhodium, lithium, and aniodide compound. The lithium can be added as lithium iodide. Aside fromthe fact that the reaction is a different one from that with which thepresent invention is concerned, there is no teaching that it isimportant per se that the lithium be present in any particular form suchas the iodide. There is no teaching that iodide ions as such aresignificant.

In summary, current technology in the field of carbonylating an alcoholsuch as methanol to form a carboxylic acid such as acetic acid stilllacks a simple method for maintaining a highly stable catalyst systemand for attaining industrially attractive conversion rates underconditions of low water content in the liquid reaction medium wherebythe expense and capital investment costs of recovering the acid productwith a very low water content can be minimized.

It is, accordingly, an object of the present invention to provide areaction system with which an alcohol, as exemplified by methanol, canbe carbonylated to a carboxylic acid derivative such as acetic acidwhile using a liquid reaction medium having a lower water content thanheretofore considered feasible. It is another object to provide acatalyst system which, regardless of the water content of the reactionmedium, will be of improved stability--i.e., more resistant toprecipitation of solid catalyst therefrom. It is also a related objectto provide a catalyst system characterized by a substantial reduction inthe undesired formation of by-product propionic acid, carbon dioxide,and hydrogen as compared with high water systems used in the prior art.Other objects will be apparent from the following detailed description.

BRIEF SUMMARY OF THE INVENTION

Broadly, the invention is an improvement in the prior-artrhodium-catalyzed carbonylation of an alcohol to produce the carboxylicacid having one carbon atom more than the alcohol. In particular, theinvention is directed to producing acetic acid (HOAc) from methanol(MeOH). Present in the reaction medium are the ester of the alcoholbeing carbonylated with the acid product of the carbonylation reactionalong with a halide derivative of the hydrocarbon corresponding to thealcohol, especially the iodide. Thus, in reaction systems whereinmethanol is being carbonylated to acetic acid, the ester is methylacetate (MeOAc) and the halide is a methyl halide, especially methyliodide (MeI). Rhodium is present in catalytically-effectiveconcentration.

The invention resides primarily in the discovery that catalyst stabilityand the productivity of the carbonylation reactor can be maintained atsurprisingly high levels, even at very low water concentrations in thereaction medium (despite the general industrial practice of maintainingapproximately 14 wt% or 15 wt% water) as discussed in EP0055618 bymaintaining in the reaction medium, along with a catalytically-effectiveamount of rhodium, at least a finite concentration of water (which can,however, be unexpectedly low as just explained) along with methylacetate and methyl iodide when making acetic acid in specifiedproportions while there is also maintained in the reaction medium aspecified concentration of iodide ions. The iodide ion, which is overand above the iodide which is present as methyl iodide or other organiciodide, is present as a simple salt, with lithium iodide beingpreferred. However, any iodide salt which is soluble in the reactionmedium in effective concentration at the reaction temperature can beemployed. No special ligands, as exemplified by, for example,phosphines, are needed.

Although the invention is broadly as just described, its preferredembodiments lie also in the discovery that there is an interactionbetween the iodide salt and the ester, especially at low waterconcentrations. That is, optimal results are obtained when each of thesenamed components is present in certain specified concentrations.Generally speaking, the iodide salt is employed in concentrations whichare higher than would be suggested by the known prior art as beingneeded. By using relatively high concentrations of the iodide salt andthe methyl ester of the acid being synthesized, one obtains a surprisingdegree of catalyst stability and reactor productivity even when theliquid reaction medium contains water in concentrations as low as about0.1 wt%, so low that it can broadly be defined simply as "a finiteconcentration" of water. The known prior art would suggest thatoperation under such low-water conditions would result in little or noformation of acetic acid. Furthermore, it has now been found that thestability of the rhodium catalyst would be very poor, especially duringthe product-recovery steps of the process wherein distillation for thepurpose of recovering the acetic acid product tends to remove from thecatalyst the carbon monoxide which, in the environment maintained in thereaction vessel itself, is a ligand with stabilizing affect on therhodium.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-25 show interactions among the several reaction mediumcomponents, which interactions are at the heart of the presentinvention.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The following description is directed to the carbonylation of methanolto produce acetic acid. However, as previously explained, the technologyis applicable to the carbonylation of higher homologues of methanol toform acids which are the higher homologues of acetic acid.

A reaction system which can be employed, within which the presentimprovement is used with no changes except for the adjustment of thecomposition of the liquid reaction medium which will be furtherexplained below, comprises (a) a liquid-phase carbonylation reactor, (b)a so-called "flasher", and (c) a "methyl iodide-acetic acid splittercolumn". The carbonylation reactor is typically a stirred autoclavewithin which the reacting liquid contents are maintained automaticallyat a constant level. Into this reactor there are continuously introducedfresh methanol, sufficient water to maintain at least a finiteconcentration of water in the reaction medium, recycled catalystsolution from the flasher base, and recycled methyl iodide and methylacetate from the overhead of the methyl iodide-acetic acid splittercolumn. Alternate distillation systems can be employed so long as theyprovide means for recovering the crude acetic acid and recycling to thereactor catalyst solution, methyl iodide, and methyl acetate. Carbonmonoxide is continuously introduced into the carbonylation reactor justbelow the agitator which is used to stir the contents. The carbonmonoxide is, of course, thoroughly dispersed through the reacting liquidby this means. A gaseous purge stream is vented from the head of thereactor to prevent buildup of gaseous by-products and to maintain a setcarbon monoxide partial pressure at a given total reactor pressure. Thetemperature of the reactor is controlled automatically, and the carbonmonoxide is introduced at a rate sufficient to maintain a constant totalreactor pressure. The carbon monoxide partial pressure in the reactor istypically about 2 to 30 atmospheres absolute, preferably about 4 to 15atmospheres absolute. Because of the partial pressure of byproducts andthe vapor pressure of the contained liquids, the total reactor pressureis from about 15 to 45 atmospheres absolute, with the reactiontemperature being approximately 150° to 250° C. Preferably, the reactortemperature is about 180° to 220° C.

Liquid product is drawn off from the carbonylation reactor at a ratesufficient to maintain a constant level therein and is introduced to theflasher at a point intermediate between the top and bottom thereof. Inthe flasher the catalyst solution is withdrawn as a base stream(predominantly acetic acid containing the rhodium and the iodide saltalong with lesser quantities of methyl acetate, methyl iodide, andwater), while the overhead of the flasher comprises largely the productacetic acid along with methyl iodide, methyl acetate, and water. Aportion of the carbon monoxide along with gaseous by-products such asmethane, hydrogen, and carbon dioxide exits the top of the flasher.

The product acetic acid drawn from the base of the methyl iodide-aceticacid splitter column (it can also be withdrawn as a side stream near thebase) is then drawn off for final purification as desired by methodswhich are obvious to those skilled in the art and which are outside thescope of the present invention. The overhead from the methyliodide-acetic acid splitter, comprising mainly methyl iodide and methylacetate, is recycled to the carbonylation reactor along with freshmethyl iodide, the fresh methyl iodide being introduced at a ratesufficient to maintain in the carbonylation reactor the desiredconcentration of methyl iodide in the liquid reaction medium. The freshmethyl iodide is needed to compensate for losses in the flasher andcarbonylation reactor vent streams.

The primary reaction control method comprises continually analyzing theliquid contents of the reactor as well as the carbon monoxide content ofthe gas in the reactor head space and, on the basis of these analyses,controlling the flow of carbon monoxide, water, methanol, and methyliodide to maintain the specified reaction medium composition. It shouldbe further explained that the methanol addition to the carbonylationreactor is based not on an analysis of its contents for methanol but,rather, on analysis for methyl acetate content. Most of the methanol isconverted almost immediately to methyl acetate when it enters thecarbonylation reactor.

There are two criteria which need to be satisfied to maintain optimalperformance of the system just described. This is over and above themaintenance of a stable catalyst system from which the rhodium catalystdoes not precipitate during the course of the flasher operation. (Aspreviously explained, this is a problem recognized by the prior artalthough the prior art has not employed the presently-described meansfor addressing it.) First, it is desired to maintain a high productivityin the carbonylation reactor itself, as measured by the quantity ofacetic acid formed per unit time per unit volume or weight of liquidreaction medium contained in the reactor. This might be termed "reactorproductivity" or "reactor space-time yield". Here again the art as itpresently exists recognizes the need to maintain reactor productivityalthough it has not taught the presently-described methods for attainingthis end.

Second, the present process improvement contemplates the maintenance ofoptimal productivity, as measured by the ultimately-recoveredconcentrated acetic acid in the combined system including both thecarbonylation reactor and the product recovery system. Although thedetails of the product recovery system, including the methyliodide-acetic acid splitter or its equivalent, are not directly relevantto the present disclosure, it will be recognized by anyone skilled inthe art that water is an undesirable component of the crude acetic acidand that the more water there is in this stream the greater will be theoperating costs and required capital investment in the productrecovery-purification system. Thus, there is also a "systemproductivity" to be considered in addition to the "reactionproductivity", with the "system productivity" depending upon the degreeto which water is kept out of the residue of the methyl iodide-aceticacid splitter column. The dryer this stream is, the higher will be theover-all system productivity so long as reaction productivity ismaintained.

The present process improvement is directed at maintaining both anoptimal reactor productivity and also an optimal over-all systemproductivity. Fundamentally, the current state of the art seems to beresigned to accepting a relatively high water content in the liquidreaction medium with a resulting undesirably high water content in thecrude acetic acid initially recovered from the reaction and primaryproduct recovery system as just described.

As previously explained, the rate of the carbonylation reactionaccording to the present state of the art has been highly dependent onwater concentration in the reaction medium as taught by U.S. Pat. No.3,769,329; EP0055618; and Hjortkjaer and Jensen(1977). That is, as thewater concentration is reduced below about 14-15 wt% water, the rate ofreaction declines. The catalyst also becomes more susceptible toinactivation and precipitation when it is present in process streams oflow carbon monoxide partial pressures. It has now been discovered,however, that increased acetic acid-production capacity can be achievedat water concentrations below about 14 wt% (at water contents aboveabout 14 wt%, the reaction rate is not particularly dependent on waterconcentration) by utilizing a synergism which exists between methylacetate and the iodide salt as exemplified by lithium iodide especiallyat low water concentrations. This effect is illustrated in Table Ibelow, which summarizes the results of five pilot plant runs in whichthe contents of the reaction medium were varied as shown with thereactor space-time yield which was attained being the criterion formeasuring efficacy of the catalyst system which was used. In each casethe space-time yield (STY) as listed is expressed in gram - moles ofacetic acid produced per hour per liter of reaction medium contained inthe carbonylation reactor, the volume of reaction medium being taken atambient temperature and in the unaerated state. The pilot plant wasoperated in the manner previously described--that is, there was astirred autoclave followed by two product recovery system distillationsteps, and the process control scheme was as described also hereinabove.The reactor temperature in all cases was between about 190° C. and 195°C. Total reactor pressure was approximately 28 atmospheres absolute,with the carbon monoxide partial pressure being approximately 8-12atmospheres absolute. In each case the balance of the liquid reactionmedium, not specifically listed in the table, was acetic acid. Minorquantities of other components were present, of course. Because thereaction rate is directly proportional to the rhodium concentration, andto facilitate the comparison of the different runs, the STY in the runsdiscussed hereinbelow has been normalized to 400 ppm rhodium unlessotherwise indicated explicitly.

                                      TABLE I                                     __________________________________________________________________________    GENERAL CONDITIONS                                                                    Run (a)                                                                              Run (b)                                                                              Run (c)                                                                              Run (d)                                                                              Run (e)                                           High Water                                                                           Low Water                                                                            Low Water                                                                            Low Water                                                                            Low Water                                         No LiI Low LiI                                                                              No LiI High LiI                                                                             High LiI                                  REAGENTS                                                                              Low MeOAc                                                                            Low MeOAc                                                                            High MeOAc                                                                           Low MeOAc                                                                            High MeOAc                                __________________________________________________________________________    Water, wt %                                                                           14     4      4      4      4                                         MeOAc, wt %                                                                           1        1-1.5                                                                              4      1      4                                         LiI, wt %                                                                             0      2.5    0      20     20                                        Rh, ppm 400    400    400    400    400                                       MeI, wt %                                                                             14     13-15  14     14     13.5                                      STY     16.9   5.2    10.4   11.0   15.8                                      __________________________________________________________________________

From inspection of the foregoing tabulation it will be seen that Run(a), with a high water content typical of the prior art, had an STY of16.9. In Run (b), with water content reduced to 4 wt% with methylacetate being slightly increased but with the other components beingessentially unchanged, an STY of only 5.2 was obtained. In Run (c) withlow water, no lithium iodide, elevated methyl acetate, and unchangedmethyl iodide, the STY was only 10.4. In Run (d), increasing the lithiumiodide content with the water being still at the low level of 4 wt%brought the STY up to a level higher than that obtained in Run (b). InRun (e), with the water still being at the relatively low level of 4wt%, an increase in both lithium iodide and methyl acetate brought theSTY up to 15.8, essentially the same STY as for Run (a) in which a highwater content was employed.

The conclusion from the foregoing comparative experiments is that underlow water concentrations methyl acetate and lithium iodide act as ratepromoters only when relatively high concentrations of each of thesecomponents are present and that the promotion is higher when both ofthese components are present simultaneously. This has not beenrecognized in the prior art. It will be seen also that the concentrationof lithium iodide was quite high as compared with what little prior artthere is dealing with the use of halide salts in reaction systems ofthis sort.

It has now also been discovered that in runs involving methyl acetateconcentration greater than about 2 wt%, lithium iodide is necessary notonly to increase the reaction rate but also to stabilize the rhodiumcatalyst due to the deleterious effect of high methyl acetateconcentrations on its stability, even at high water concentrations. Forexample, in experimentation carried out at 200° C. with 14 wt% water inthe reaction medium along with 15 wt% methyl iodide and no lithiumiodide and using 320-240 ppm of rhodium as the catalyst, the rhodiumprecipitation loss was found to be about 12 ppm of rhodium concentrationper hour at an average concentration of 2 wt% methyl acetate in thereaction medium whereas, with other reaction components beingsubstantially unchanged, the rhodium loss was 1.3 ppm per hour or lowerwhen the methyl acetate content was only about 1 wt%. This exemplifiesagain that the reaction-accelerating effect of methyl acetate is bestrealized in conjunction with a relatively high concentration of iodidesalt. This has not been recognized in the prior art.

Some runs were made in which the reaction was carried out in a batchautoclave instead of the continuously-operating pilot plant reactionsystem as described above. In these runs an autoclave of suitablycorrosion-resistant metal was charged with rhodium triiodide (typicallybetween 200 and 500 ppm rhodium content in the resulting mixture), 14 to19 wt% methyl iodide, water in the concentration that was to be tested,variable amounts of the stabilizer which was to be tested, 15 ml ofmethanol, and 40 to 60 grams of acetic acid. The autoclave was sealed,pressured to approximately 28.2 atmospheres absolute of carbon monoxidepartial pressure and pressure checked at 25° C. After this the autoclavewas slowly vented of its carbon monoxide content and then flushed twotimes with 4.4 atmospheres absolute of carbon monoxide. The autoclavewas then pressured to 11.2 atmospheres absolute with carbon monoxide andheated to 185° C. to 190° C., after which the agitator with which theautoclave was provided was turned on. The autoclave was then furtherpressured with carbon monoxide to 28.4 atmospheres absolute, and therate of reaction was determined by monitoring the amount of carbonmonoxide consumed over a period of time while assuming that theideal-gas law applied to carbon monoxide. Reaction rate was determinedfrom plots of carbon monoxide uptake versus time, the resulting datathen being converted to the carbonylation reaction rate assuming idealgas behavior for the carbon monoxide. This procedure was generally usedin studying the effect of using as reaction stabilizer several iodidesalts, some of which had organic cations.

Using both the continuous pilot plant and also the batch reaction systemas just described, it has now been determined that the interactionbetween water content, iodide salt, methyl acetate, and methyl iodide isas set forth in the following tabulation, in which there are set forthboth a broad range and a preferred, or optimal, range for obtaining bothcatalyst stabilization and reaction rate enhancement. The "preferred"range is that which is preferred from the standpoint of optimalperformance of the entire system including the primary product recoverysystem as explained hereinabove. It will be seen that the recommendedconcentrations are the same for both stabilization and also rateenhancement with one exception: the exception is that the "preferred"range for methyl acetate is 0.5-5 wt% for

catalyst stabilization whereas it is 2-5 wt% for optimal rateenhancement. Broadly, of course, this means that in either case a rangebetween 0.5 wt% and 5 wt% would be satisfactory, but that, dependingupon whether it is catalyst stabilization or maximal rate enhancementthat one aims to maximize in a given plant operating situation, thebottom end of the desired methyl acetate range is slightly higher whenmaximal rate enhancement is being sought.

                  TABLE II                                                        ______________________________________                                                              RATE                                                             Stabilization                                                                              ENHANCEMENT                                                      Broad   Preferred                                                                              Broad     Preferred                                          wt %    wt %     wt %      wt %                                      ______________________________________                                        H.sub.2 O  .sup.(1) 0.1-20.sup.                                                                    1-4      .sup.(1) 0.1-20.sup.                                                                  1-4                                     Inorganic   2-20     10-20     2-20   10-20                                   Iodide (as LiI)                                                               MeOAc      0.5-30    0.5-5    0.5-30  2-5                                     MeI         5-20     12-16     5-20   12-16                                   HOAc       Balance   Balance  Balance Balance                                 Rh (ppm)    200-1000 300-600   200-1000                                                                             300-600                                 ______________________________________                                         .sup.(1) Particular utility obtains at about 0.1-14%, water content being     a more significant factor below about 14 wt %.                           

To reiterate what has been said hereinabove, water contents below about14% are low as compared with prior art, and the iodide salt content hereis quite high. The upper end of the recommended methyl acetateconcentration is also higher than one can calculate as being present ina simulated commercial catalyst solution (EP0055618).

The interrelationship between lithium iodide concentration and watercontent in the reaction medium was investigated in a series of batchruns in which lithium iodide content in the reaction medium was variedbetween about 0.0 molar and about 1.5 molar (20 wt%) with 2 wt% water inthe reaction medium, the results so obtained being compared with thoseobtained with 14 wt% water in the medium. Methyl iodide concentrationwas 14 wt%, reaction temperature was 190° C., and rhodium content of thereaction medium was 472 ppm. The initial methyl acetate content was 27wt% in these batch runs. In continuous operation it would be much lower.With 14 wt% water, the space-time yield declined, as the lithium iodidecontent declined, from about 20 moles per liter per hour at about 1.5molar (20 wt%) lithium iodide concentration down to about 12 to 13 molesper liter per hour with a lithium iodide molar concentration of about0.8 (11 wt%). There was some scatter of data points, and with no lithiumiodide at all, the space-time yield was indicated to be about 13. Thecurves of rate versus lithium iodide concentration were not as welldefined at high water as at 2 wt% water.

With 2 wt% water, the effect of lithium iodide was pronounced. At around0.2 molar (2.7 wt%) lithium iodide, the space - time yield was 7 molesper liter per hour, and this increased with lithium iodide increase in avery nearly linear fashion to a space-time yield of about 21 moles perliter per hour when the lithium iodide concentration was about 1.5 molar(20 wt%). Thus, by increasing lithium iodide content it was possible toobtain substantially the same space-time yield at 2 wt% water as at 14wt% water with a pronounced resulting enhancement of the ability of theplant to operate under desirable conditions of low water content.

The interrelation between methyl acetate and lithium iodide content wasinvestigated in three sets of batch runs in which, at a constant lithiumiodide content in each case, the methyl acetate content of the reactionmedium was varied from 0 to a maximum of about 3.0 molar (33 wt%). Inall cases the methyl iodide content was 14 wt%, the water content was 2wt%, the temperature was 190° C., and the rhodium content was 236 ppm.When the lithium iodide content was 0.17 molar (2.5 wt%), the space-timeyield increased gradually from 0 when no methyl acetate was present upto about 7 moles per liter per hour when the methyl acetate content wasabout 26 wt%. Plotted on rectangular coordinates, the curve was gentlyconvex upward. When the lithium iodide content was 1.5 molar (20 wt%),the space-time yield increased from 0 when the methyl acetate was 0 toabout 14 moles per liter per hour when the methyl acetate content wasabout 33 wt%. That is, when the methyl acetate was about 33 wt%, the useof 1.5 molar (20 wt%) lithium iodide multiplied the space-time yield bya factor of about 2 as compared with conditions obtained when using 0.17molar (2.5 wt%) lithium iodide.

Another series of runs was carried out to investigate the differences,if any, between lithium iodide (a representative metal iodide salt) andN-methylpicolinium iodide (NMPI), a representative salt having anorganic cation. NMPI is formed by quaternizing 3-picoline with methyliodide. The reaction medium contained the NMPI, 2 wt% water, 14.4 wt%free methyl iodide, 27 wt% methyl acetate, and the balance acetic acid.It also contained 472 ppm of rhodium. Reaction temperature was 190° C.Over a concentration range of either lithium iodide or NMPI ranging fromabout 0.2 molar to about 0.8 molar, a plot of reaction space-time yieldagainst the molar concentration of either the lithium iodide or the NMPIshowed that, within the limits of experimental error, there was nodifference in space-time yield obtained at a given molar concentrationof lithium iodide as compared with the same concentration of NMPI. Itwill be recognized that it is the concentration of iodide ion that isthe controlling factor, and that at a given molar concentration ofiodide the nature of the cation is not as significant as the effect ofthe iodide concentration. Any metal iodide salt, or any iodide salt ofan organic cation, can be used provided that the salt is sufficientlysoluble in the reaction medium to provide the desired level of thestabilizing iodide. The iodide salt can be a quaternary salt of anorganic cation or the iodide salt of an inorganic cation, preferably itis an iodide salt of a member of the group consisting of the metals ofGroup Ia and Group IIa of the periodic table as set forth in the"Handbook of Chemistry and Physics" published by CRC Press, Cleveland,Ohio, 1975-76 (56th edition). In particular, alkali metal iodides areuseful, with lithium iodide being preferred.

EXAMPLE 1

The following run was carried out in continuously-operating apparatuscomprising a stirred reactor from which the product was drawn offcontinuously for workup in the manner previously described hereinabove.The carbonylation reactor contained approximately 1800 ml of liquidreaction medium, measured at ambient temperature in the bubble-freestate. Its contents were analyzed periodically throughout the run, andthese analyses were employed to control the flows of the several streamsentering the reactor in such a manner as to maintain in the liquidreaction medium about 13 to 16 wt% methyl iodide, 4 to 5 wt% methylacetate, 19 to 19.5 wt% lithium iodide, 4 to 5 wt % water, and 310 to335 ppm of rhodium. The balance of the reaction medium was acetic acid.Before starting the run, the carbonylation reactor had been initiallycharged with a mixture of about 16 wt% water, 12 wt% methyl iodide, 0.7wt% methyl acetate, and the balance acetic acid, the total mixturecontaining about 400 ppm of rhodium in the form of a rhodium carbonyliodide compound. The rhodium compound can be prepared by dissolvingrhodium triiodide in acetic acid containing 15-20 wt% water at about110° C. while sparging carbon monoxide through the mixture at a pressureof about one atmosphere absolute or higher.

During operation the reactor temperature was maintained between about189° C. and 191° C. The pressure was maintained at about 28 atmospheresabsolute. Carbon monoxide was continuously introduced through a spargersituated below the agitator blades, and a continuous vent of gas wasdrawn off from the top of the vapor space contained in the upper part ofthe reactor at about 15 liters per hour (ambient temperature andpressure). The carbon monoxide partial pressure in the reactor headspace was maintained at about 13 atmospheres absolute.

By means of a level control sensing the liquid level within the reactor,liquid reaction product was continuously drawn off and fed onto the trayof a single-tray flasher operating at a head pressure of about 2.4atmospheres absolute. Of the liquid fed into the flasher, approximately35% was distilled overhead for further redistillation in the methyliodide-acetic acid splitter column while the remainder was drawn fromthe base of the column and returned to the carbonylation reactor. Thisstream comprised predominantly acetic acid and contained the catalyst.

The methyl iodide-acetic acid splitter column contained 20 trays, withthe overhead from the flasher just described being introduced onto the15th tray from the bottom. This splitter column was operated at a headpressure of 1 atmosphere absolute and with a reflux ratio of 1:1. Of thefeed initially introduced into this column, approximately 60% was takenoverhead and was recycled to the carbonylation reactor. This streamcontained predominantly methyl iodide and lesser quantities of methylacetate. Such methyl iodide makeup as was necessary to maintain thedesired methyl iodide content in the carbonylation reactor wasintroduced into this recycling stream before it was returned to thecarbonylation reactor. The rate of methyl iodide introduction was set byperiodic analyses of the vent streams leaving the reactor and theflasher, enough methyl iodide being introduced to make up for theseprocess losses. Also introduced into this stream just before enteringthe carbonylation reactor was sufficient methanol to maintain thedesired methyl acetate content in the reactor liquid medium. (Methanolis converted almost immediately to methyl acetate upon entering thereactor). Such water as was needed to maintain the desired water contentin the reactor was also introduced with this methyl iodide recyclestream.

Preferably, water recovered in any of the distillate streams is recycledto the reactor. There is very little consumption of water in thereaction. If a water phase forms at any point in the product-recoverysystem, it will probably contain methyl iodide, which should be returnedto the reactor.

The residue stream from the methyl iodide-acetic acid splitter columnwas drawn off as the crude acetic acid product, to be purified furtheras desired by conventional methods outside the scope of the presentinvention. As previously explained, a primary object of the operationwas to produce a crude acetic acid at this point containing only a smallamount of water.

With the system operating as just described, the STY of acetic acid inthe crude acetic acid product drawn from the base of the methyliodide-acetic acid splitter was approximately 14 gram-moles of aceticacid (calculated as pure acetic acid) per hour per liter of liquidreaction medium contained in the carbonylation reactor, the volume ofsaid liquid reaction medium being measured at ambient temperature. Thewater content of the crude acetic acid was approximately 4 to 7 wt%.This is to be compared with a water content of 20 to 25 wt% and an STYof 13 with the same rhodium concentration where, in accordance with theusual practice of the prior art, the carbonylation reactor was operatedwith a water content of approximately 15 wt% in the reaction medium.

As indicated by periodic analyses of the contents of the carbonylationreactor, there was very little precipitation of catalyst from thereaction medium in the flasher column and the transfer lines recyclingthe catalyst solution from this column back to the carbonylationreactor, although our experience with solutions without iodide salts asin the prior art would have led one to predict a serious catalyst-lossproblem.

When using other iodide salts, the controlling factor is theconcentration of iodide moiety supplied by whatever salt is employed.That is, the beneficial results obtained with a given concentration oflithium iodide will also be obtained with other iodide salts when theyare used in a concentration such that the molar equivalent iodideconcentration is the same as that obtaining with a given lithium iodideconcentration known to be effective.

An unexpected effect of operating the reaction system by the low-watermethod just described is also that there is a great reduction (by anorder of magnitude) in the rate of formation of by-product propionicacid, the presence of which in the product acetic acid is objectionablefor several reasons. Again as compared with the relatively high-wateroperating conditions of the prior art, there is a substantial reductionin the rate of formation of hydrogen and carbon dioxide, which, ofcourse, are undesirable reaction products. These are formed by thewater-gas shift reaction from carbon monoxide and water. The followingtabulation compares yields of propionic acid (HOPr), carbon dioxide, andhydrogen obtained at the above conditions of 4 to 5 wt% water with thoseobtained using 14 to 15 wt% water in the reaction system characteristicof the prior art (no iodide salt). Methyl acetate content of thereaction medium was about 1 wt% in the high water medium and about 4 wt%in the low water system.

                                      TABLE III                                   __________________________________________________________________________           CO.sub.2 Make                                                                          H.sub.2 Make Acetic Acid                                      Reactor                                                                              (Moles CO.sub.2 /100                                                                   (Moles H.sub.2 /100                                                                   HOPr % Yield Based                                    H.sub.2 O                                                                            moles HOAc)                                                                            moles HOAc)                                                                           (ppm)                                                                              on MEOH                                          __________________________________________________________________________    14-15% 2.3      1.9     1435.sup.(1)                                                                       99.sup.(2)                                       (No iodide                                                                    salt)                                                                         4-5%   0.2      0.1      91.sup.(1)                                                                        99.sup.(2)                                       (Iodide salt                                                                  as described                                                                  above)                                                                        __________________________________________________________________________     .sup.(1) In acid product from base of MeIHOAc splitter.                       .sup.(2) Approximate, within experimental margin of error. As calculated,     yield was slightly higher in the "low water" case.                       

EXAMPLE 2

Other iodide salts are as efficacious as lithium iodide at the sameiodide moiety concentration in the reaction medium. For example, in thecontinuous reaction system described in Example 1 a run was made inwhich the iodide salt was sodium iodide. Operating in the same manner asdescribed with lithium iodide in Example 1, but with the iodideconcentration being reduced because of the limited solubility of sodiumiodide as compared with lithium iodide, the run was made underconditions as set forth in Table IV below. The reaction medium was astabulated below, with acetic acid making up the balance in eachtabulated case.

The results as tabulated show that, at the same concentration of iodidemoiety, sodium iodide gave results as good as those obtained withlithium iodide Specifically, within the indicated limits of accuracy,results were identical. When using the higher water concentrationcharacteristic of the prior art but with no iodide salt, the acetic acidspace-time yield was slightly higher, but it is to be kept in mind thatthis was at the expense of having to work in the recovery system with acrude reaction medium containing 14 wt% water instead of 4 wt%. It isalso to be kept in mind that in actual application of the presentinvention the iodide concentration would have preferably been higherthan the indicated 9.4 wt%, which was the maximum concentration whichcould be used in the present Example in order to maintain comparabilitywith sodium iodide, the solubility characteristics of which precludedusing the higher concentrations which would actually be preferred.

                  TABLE IV                                                        ______________________________________                                                         Promoter/Stabilizer                                                           Iodide Salt                                                                   NaI   LiI                                                    ______________________________________                                        Inorganic Iodide (wt %)                                                                          9.5     9.4                                                Temperature (°C.)                                                                         190     190                                                Water, (wt %)      4.0     4.0                                                Methyl Iodide (wt %)                                                                             12.2    12.1                                               Methyl Acetate (wt %)                                                                            3.1     3.1                                                Rhodium (ppm)      400     400                                                Acetic Acid STY    14.3    12.7                                               (mol/l · hr)                                                         Carbon Dioxide STY 0.39    0.35                                               (mol/l · hr)                                                         Propionic Make Rate                                                                              150     109                                                (lb/MM lb acetic acid)                                                        Rhodium Loss, (ppm/hr)                                                                           0.75    0.73                                               ______________________________________                                    

The effect of using a variety of iodide salts is set forth in Table Vbelow. These data are all from runs which were carried out in the batchautoclave operated in the manner previously described. These dataindicate that other iodide salts have a rate acceleration (promoting)action as well as does lithium iodide. FIG. 9 shows stabilizing actionof several specific iodides. However, many of these do not have a veryhigh solubility when the reaction medium is cooled much below normaloperating temperature. Lithium iodide continues to be preferred becauseof its superior solubility characteristics.

                  TABLE V                                                         ______________________________________                                        Rate of Methanol Carbonylation With Various Iodide Sources                    Batch Autoclave                                                               Charge: 19 wt % MeI, 472 ppm Rh, 27 wt % MeOAc, 0.75 M I.sup.-                (equiv. to 10 wt % Li) 28.2 atm. abs., 190° C.                                      2 wt %  4-5 wt %                                                              H.sub.2 O                                                                             H.sub.2 O                                                Salt         STY     STY        Comments                                      ______________________________________                                        no salt      3.0     10.9                                                     LiI          12.2    14.8       Soluble                                       NaI          8.8     --         soluble                                       KI           11.2    13.2       partially soluble                             RbI          --      4.3        poor solubility                               CsI          --      --         insoluble                                     MgI.sub.2    10.7    12.7       partially soluble                             CaI.sub.2    17.2    --         soluble                                       SrI.sub.2    7.0     --         soluble                                       BaI.sub.2    11.2    15.9       soluble                                       CoI.sub.2    12.6    --         soluble                                       SbI.sub.3    --      --         insoluble                                     ZnI.sub.2    5.1     11.5       soluble                                       SnI.sub.2    1.3     --         soluble                                       FeI.sub.2    3.8     13.5       partially soluble                             LaI.sub.3    --      16.7       partially soluble                             NiI.sub.2    --      3.5        insoluble                                     MnI.sub.2    8.9     --         soluble                                       NMPI         10.1    --         soluble                                       (Ph) (CH.sub. 3).sub.3 N.sup.+ I.sup.-                                                     6.1     --         partially soluble                             Bu.sub.4 N.sup.+ I.sup.-                                                                   7.1     --         soluble                                       (Et) (Ph).sub.3 P.sup.+ I.sup.-                                                            8.9     --         soluble                                       NH.sub.4.sup.+ I.sup.-                                                                     4.67    --         insoluble                                     ______________________________________                                    

It will be understood that the foregoing Examples are given merely byway of illustration and that many departures can be made therefromwithin the scope of the invention. In particular it will be understoodthat the hear of the invention lies in controlling the carbonylationreactor itself so as to produce a product mixture having a low watercontent as compared with the prior art while avoiding losses in reactorproductivity. The product-recovery system exemplified above is onewhich, while industrially applicable, was especially selected for easeof control while studying and demonstrating the invention. It will beobvious to those skilled in distillation that, to divide the drawn-offcarbonylation reaction medium into a recycle catalyst stream, a crudeacetic acid product stream, and a recycle or recycles comprising methyliodide and methyl acetate, many alternatives are easily foreseable amongwhich the process designer can select what he views as the optimum forreliable and economical operation in his own circumstances.

The drawings FIGS. 1-25 describe the interaction of the several processparameters the manipulations of which is important in the practice ofthe present invention. Some of these figures set forth the results ofruns carried out in the batch autoclave (operation previously describedherein), some present the results of runs carried out in the continuouspilot plant unit (operation also described previously herein), and someare based on results obtained in a batch-operated glass vessel which wasdesigned specifically to study catalyst stability. This vessel wasactually composed of two side-by-side vessels fabricated from glass pipeand designed to operate at pressures not to exceed about 2 atmospheresgauge pressure at 150° C. To conduct a run, each of the glass vesselswas initially charged with the desired weight of rhodium (as salts likeRhI₃), HI, acetic acid, water, and stabilizer. Both vessels were thenpressurized to about 1.8 atmospheres gauge with carbon monoxide andheated in an oil bath to 130° C. or 150° C. in order to dissolve therhodium. Carbon monoxide was then bubbled into the solution at 47 ml perminute through a gas-inlet tube while the desired constant pressure wasmaintained by a back-pressure regulator system. After one hour, thecarbon monoxide was replaced by nitrogen and the total pressure wasreduced to about 1 atmosphere gauge. This was considered the initialtime of the stability experiment. Samples were removed through asampling port, centrifuged for 5-10 minutes, and the clear centrifugateanalyzed for soluble rhodium content.

Turning now to the information set forth in the drawings and consideringthe drawings in numerical order:

FIGS. 1 through 9 show the results of batch experiments. FIG. 1illustrates that reducing the water content of the reaction system doesreduce the reaction space-time yield, but that with high lithium iodidein the reaction medium along with high methyl acetate and methyl iodide,good carbonylation rates can be obtained at surprisingly low waterconcentrations. It also shows the agreement of data obtained in batchautoclave and the continuous unit. FIG. 2 illustrates that space-timeyield increases with increasing lithium iodide concentration. Althoughthere is some scatter in the data especially at high waterconcentration, it is also indicated that increasing the lithium iodideconcentration mitigates what would otherwise be the adverse effect onreaction rate of reducing the water concentration. The effect of iodideat low water (2 wt%) is very well defined and impressive.

FIG. 3 demonstrates that the methyl acetate concentration is asignificant factor and that it is inter-related with the employment ofthe lithium iodide stablizer. Both with and without lithium iodide beingpresent, increasing the methyl acetate concentration up to somewhat lessthan 10 wt% increases the space-time yield, but with 20% lithium iodidebeing in the reaction medium the space-time yield at a given methylacetate concentration is roughly double that observed when the lithiumiodide is not present even at lower water concentration.

FIG. 4 illustrates the significance of methyl iodide concentration inthe reaction medium with varying lithium iodide concentration. With nolithium iodide, space-time yield increases with increasing methyl iodideconcentration but the space-time yields are relatively low. With 2.5 wt%lithium iodide in the mixture the space-time yields are higher than withnone, still, however, showing a methyl iodide dependency. With 11 wt%lithium iodide the space-time yields are even higher, still showing anincrease with increasing methyl iodide.

FIG. 5 demonstrates, not surprisingly, that the space-time yieldincreases with increasing rhodium concentration in the reaction medium.It is further demonstrated, however, that results are poorest when thereis no lithium iodide present, better when there is 2 5 wt% lithiumiodide, and (within the range illustrated here) best when the lithiumiodide concentration is 14 wt%.

FIG. 6 illustrates that increasing water in the reaction mediumdecreases the rate of rhodium catalyst precipitation. Also illustratedin FIG. 6, an increase in iodide moiety by adding lithium iodide reducesthe rate of rhodium precipitation out of the reaction medium at a givenhydrogen iodide and water concentration. FIG. 7 illustrates thestabilizing effect of lithium iodide at low (3 wt%) water concentrationand at two temperatures (130° C. and 150° C.) At the lower temperature,roughly 6 wt% lithium iodide results in catalyst stability as good asthat obtained when using a reaction medium containing 15 wt% water andneeding no stablizer. At the higher temperature, about 15 wt% lithiumiodide is adequate. In FIG. 8 it is demonstrated that, in the absence oflithium iodide, very little rhodium remains in solution after 8 hours orless in a reaction medium of the composition described.

FIG. 9, based on data obtained in the batch autoclave, illustrates thatit is the halide (in this case iodide) moiety which is the significantfactor in stabilizing the reaction catalyst. Note especially, forexample, that at about 0.28 molar concentration of iodide the (low)rhodium loss per hour is essentially the same regardless of the sourceof the iodide.

FIG. 10, as well as FIGS. 11-25, presents data taken from the continuousunit the operation of which has been previously described. FIG. 10itself illustrates that high lithium iodide together with high methylacetate counteracts the deleterious effects on space-time yield ofreducing the water concentration in the reaction medium. It will be seenthat with 16 to 21 wt% lithium iodide and 4 wt% methyl acetate thespace-time yields obtainable at 2 wt% water in the reaction medium arealmost as good as those obtained at higher water concentrations ofaround, for example, 10 wt% with 1 wt% methyl acetate and 0-2.5 wt%lithium iodide. It should be explained, incidentally, that for datapoints at 4 wt% methyl acetate conditions set out in FIG. 10 there is arange of lithium iodide concentration. This is due to the fact that thesteady state lithium iodide content is determined by an equilibriumbetween lithium iodide and lithium acetate which is affected by thechange in reactor water and methyl acetate content. This will be shownlater (FIG. 20). This is also true for similar figures to follow.

FIG. 11 illustrates that the reaction rate is dependent on waterconcentration even at high concentrations of lithium iodide, but that atabout 1 wt% water the use of high lithium iodide brings the reactionrate up to about 10 to 12 moles per liter-hour and that above about 2wt% water the use of high lithium iodide brings about space-time yieldsalmost as high as those obtained at 8 wt% water and higher (FIG. 10).

FIGS. 12 and 13 describe the effect of increasing lithium iodideconcentration in increasing the space-time yield of acetic acid at twolevels of methyl acetate in the reaction medium. These data, which arefrom the continuous unit, can be read in conjunction with FIG. 2, whichpresents data from the batch autoclave.

The effect of lithium iodide on the rate of methanol carbonylation underconditions of high water (8 wt%) and low methyl acetate (1 wt%)concentration as shown in FIG. 13 would appear to be relatively small inthe range of 0-20 wt% lithium iodide (ca. 18% rate increase) whencompared with FIG. 12 and also with FIG. 2 (batch). The differences aremainly due to the different methyl acetate and water concentrations usedin the runs in the different figures. The higher the methyl acetate andthe lower the water concentration the higher is the effect of lithiumiodide on the rate. Because lithium iodide stabilizes the Rh catalyst,it becomes possible to decrease the reactor water concentration in orderto increase throughput in the purification train. Also if the waterconcentration is decreased in conjunction with increasing the methylacetate concentration, a significant rate enhancement due to lithiumiodide is observed as shown in FIG. 12 (4 wt% water, 4 wt% methylacetate, 0-21 wt% lithium iodide; 23-50% rate increase from 0-21 wt%lithium iodide) and in FIG. 2 (2-8 wt% water, 27 wt% methyl acetate and2-20% lithium iodide, 200% rate increase from 2-20 wt% lithium iodide).Therefore, lithium iodide addition makes possible operation in a newconcentration range of low water and high methyl acetate (FIG. 10),heretofore impossible because of low rates and severe catalystinstability. Further evidence for rate enhancement due to lithium iodideis given in FIG. 2 which shows that the lower the water concentrationand the higher the methyl acetate concentration the greater therate-enhancing effect of lithium iodide.

The effect of methyl acetate (in the presence of high lithium iodideconcentrations)on the acetic acid space-time yield is shown in FIGS. 14and 15. In both cases the effect of adding methyl acetate is beneficialup to a level of about 4 to 5 wt%, after which the effect levels off or(FIG. 15) declines slightly. Between 0 and about 3 wt%, the beneficialeffect of adding methyl acetate is marked. Using 20 wt% lithium iodideis seen to be more beneficial than using 10 wt%, and space-time yield issomewhat better with 8 wt% water as compared with 4 wt%.

FIGS. 16 and 17 show that the acetic and space-time yield increases whenincreasing methyl iodide concentration and rhodium concentrationrespectively, as expected.

FIG. 18 illustrates the effect of lithium iodide, methyl acetate, andwater on the (undesired) formation of carbon dioxide as a reactionby-product. When using 16 to 21 wt% lithium iodide and 4 wt% methylacetate the generation of carbon dioxide is much lower than when using 0to 2.5 wt% lithium iodide and only 1 wt% methyl acetate. It is also tobe noted that reducing the water content with a given reaction mediumhas the effect of reducing the rate of formation of carbon dioxide.Reducing carbon dioxide formation in this manner, by using the lithiumiodide or equivalent stabilizers of the present invention, is anotherunexpected result of operating in the low-water reaction medium the useof which is made possible by employing these stabilizers. FIGS. 19, 20,21, and 22 further show the individual effects of lithium iodide, methylacetate, and methyl iodide at low water concentration (4 to 8 wt%) onthe formation of carbon dioxide. FIG. 20 also shows the equilibriumconcentration of hydrogen iodide at various lithium iodideconcentrations.

FIG. 23 deals with the equilibrium existing in the reaction mediumbetween lithium iodide and lithium acetate:

    LiI+MeOAc⃡LiOAc+MeI

with decreasing water content the lithium acetate content of thereaction medium increases, this effect being greater when 12 wt% methylacetate is present as compared with 4 wt%. This equilibration betweenlithium iodide and lithium acetate which is dependent on the waterconcentration of the reaction medium has been found, incidentally, tohave no adverse effect on the behavior of the catalyst system. As amatter of fact this equilibrium will allow the increasing of the lithiumiodide concentration of the reaction medium by adding, if desired,lithium acetate or other lithium salts. Because of this equilibrium onecannot distinguish the effect of lithium iodide from that of lithiumacetate on the reaction rate and it is possible that both the lithiumiodide and lithium acetate increase the reaction rate, especially withcatalyst solutions with low water concentration. However, the importantfact is that adding either lithium acetate or lithium iodide one obtainseventually the same equilibrium mixture of both salts in solution.

FIGS. 24 and 25 depict the results of studies of rhodium loss from thereaction medium in the continuous unit, FIG. 24 demonstrating thatincreasing the lithium iodide concentration greatly reduces rhodium lossat varying water concentrations and at two different methyl acetateconcentrations while FIG. 25 demonstrates that at higher waterconcentrations there is less rhodium loss and also that going to therelatively high methyl acetate concentration of 12 wt% increases rhodiumloss as compared with using 4 wt% methyl acetate.

The embodiments of the invention in which an exclusive property orprivilege is claimed are:
 1. In a process for producing acetic acid byreacting methanol with carbon monoxide in a liquid reaction mediumcontaining a rhodium catalyst and comprising water, acetic acid, methyliodide, and methyl acetate and subsequently recovering acetic acid fromthe resulting reaction product, the improvement whichcomprises:maintaining catalyst stability and system productivity bymaintaining in said reaction medium during the course of said reactionabout 0.1 wt% to less than 14 wt% of water together with (a) aneffective amount in the range of about 2 wt% to 20 wt% of a catalyststabilizer selected from the group consisting of iodide salts which aresoluble in said reaction medium in effective concentration at reactiontemperature, (b) about 5 wt% to 20 wt% of methyl iodide, and (c) about0.5 wt% to 30 wt% of methyl acetate.
 2. The process of claim 1 whereinsaid iodide salt is a quaternary iodide salt or an iodide salt of amember of the group consisting of the metals of Group Ia and Group IIaof the periodic table.
 3. The process of claim 2 wherein said iodidesalt is an alkali metal iodide.
 4. The process of claim 3 wherein theiodide salt is lithium iodide.
 5. The process of claim 4 wherein therhodium catalyst is maintained in said reaction medium in aconcentration of about 200 ppm to about 1000 ppm calculated as rhodium.6. The process of claim 5 wherein there is maintained in the reactionmedium about 1 to less than 14 wt% water, 10 to 20 wt% lithium iodide,12 to 16 wt% methyl iodide, and 0.5 to 5 wt% methyl acetate, with thebalance consisting essentially of acetic acid.
 7. The process of claim 1comprising maintaining in said reaction medium during the course of saidreaction about 1 wt.% to about 12 wt.% water.
 8. The process of claim 1comprising maintaining in said reaction medium during the course of saidreaction about 1 wt.% to about 10 wt.% water.
 9. The process of claims1, 7 or 8 wherein said iodide salt is lithium iodide and maintaining insaid reaction medium during the course of said reaction 10 to 20 wt.%lithium iodide, 0.5 to 5 wt.% methyl acetate and 10 to 16 wt.% methyliodide, with the balance consisting essentially of acetic acid.
 10. Theprocess of claim 9 comprising maintaining in said reaction medium duringthe course of said reaction 2 to 5 wt.% of said methyl acetate.
 11. Theprocess of claim 9 comprising maintaining in said reaction medium duringthe course of said reaction about 12 to about 16 wt.% of said methyliodide.
 12. In a process for producing acetic acid by reacting methanolwith carbon monoxide in a liquid reaction medium containing a rhodiumcatalyst and comprising water, acetic acid, methyl iodide, and methylacetate and subsequently recovering acetic acid from the resultingreaction produce, the improvement which comprises:maintaining catalyststability and system productivity by maintaining in said reaction mediumduring the course of said reaction about 0.1 wt.% to less than 14 wt.%of water together with (a) about 2 wt.% to 20 wt.% of a catalyststabilizer selected from the group consisting of lithium iodide, lithiumacetate, and mixtures thereof, (b) about 5 wt.% to 20 wt.% of methyliodide, and (c) about 0.5 wt.% to 30 wt.% of methyl acetate.
 13. Theprocess of claim 12 comprising maintaining in said reaction mediumduring the course of said reaction about 1 wt.% to about 12 wt.% water.14. The process of claim 12 comprising maintaining in said reactionmedium during the course of said reaction about 1 wt.% to about 10 wt.%water.
 15. The process of claims 12, 17 or 18 comprising maintaining insaid reaction medium during the course of said reaction 10 to 20 wt.% ofsaid catalyst stabilizer, 10 to 16 wt.% of said methyl iodide and 0.5 to5 wt.% of said methyl acetate, with the balance consisting essentiallyof acetic acid.
 16. The process of claim 15 comprising maintaining insaid reaction medium during the course of said reaction 2 to 5 wt.% ofsaid methyl acetate.